For some time, the engineering community has been challenged by the impracticality of a silicon-based device to meet ongoing demands for emitting and receiving light of certain wavelengths in various applications. These challenges have been heightened due to a tremendous and relatively recent increased interest in silicon-based photonics. Both the computing and the telecommunications industry have struggled to develop electrical interconnects for short distances, at a pace that is commensurate with an ever-increasing demand for communication bandwidth. Such development is hampered by limitations inherent to electrical interconnects and due to the enormous complexity and power required to achieve high communication bandwidth. A major impediment to an all-silicon photonics solution is the implementation of a monolithic silicon and CMOS compatible light-emitter including a laser. While there are a number of commercially-available flip-chip bondable lasing devices that can, in principle, enable optical interconnects, such devices render relatively low bonding yields that result in overall high chip costs, thereby presenting a significant barrier for replacement of electrical interconnects by optical interconnects.
In many applications, technological advancements are practicably realized by implementing new circuit elements/arrangements directly on silicon, for example, using a monolithic CMOS compatible process. Such an implementation enables the technologies to ride on a multi-billion dollar silicon manufacturing infrastructure, accompanied by the inherent volume nimbleness, and mitigation of capital/operational expenses through sharing with other CMOS IC companies. For development or growth directly on silicon, materials need to be compatible. Since silicon is an indirect bandgap material, the radiative recombination efficiency for light emission is very low compared to direct bandgap III-V materials. Other silicon compatible materials, such as Germanium (Ge), have recently become common in a mainstream CMOS fabrication facility, at least in low concentration. Further, Ge is rigorously being researched because of its superior electrical properties. However, Ge in its bulk, unstrained form, is also an indirect bandgap material, and suffers from some of the same light emission inefficiency problems as silicon.
Apart from light emission and laser, other key components of silicon-photonics are an efficient light detector, light modulator, and light amplifier. For a light/photodetector, there is a strong need to have a highly-responsive, efficient photodetector that also meets two important criteria. First, the photodetector should be compatible with the telecommunication standards, which operate at a wavelength near 1550 nm. Second, the photodetector should be compatible with silicon manufacturing processes, so that it can be produced inexpensively. Light absorption required for detection is altogether absent in silicon around 1550 nm wavelengths, thus rendering it unworthy as a detector at these wavelengths. To the extent that the silicon IC industry would progressively use more Ge in the form of SiGe compounds for transistor mobility enhancement, the variety and importance for these applications can increase dramatically. Ge by itself can serve as an excellent photodetector at 1310 nm wavelength (with an absorption of ˜7000 cm−1 at 1310 nm), but it is a poor detector at 1550 nm with an absorption coefficient of only around 450 cm−1. Optical devices, such as detectors and modulators, would benefit from reduction in the bandgap of Ge regardless of whether the bandgap was indirect or direct. For instance, a reduction in bandgap can improve the detection of low energy photons due to the smaller amount of energy necessary to move electrons between the valence band and the conduction band.
Several different types of photodetectors exist, including (but not limited to) metal semiconductor metal (MSM), PIN, and waveguide detectors. A photodetector can be evaluated based on its: 1) responsivity or quantum efficiency (which can impact signal-to-noise ratio), 2) the flatness or uniformity of its response to wavelengths around a central wavelength, for example, around the commonly deployed communication wavelength of 1550 nm, 3) transit time (which can impact device speed), 4) capacitance (which impacts the bandwidth, the signal-to-noise ratio at the front-end and power consumption), 5) bandwidth, 6) dark current (important to signal-to-noise ratio and power dissipation), 7) power dissipation, and 8) area. In detector designs, there is typically a tradeoff between these performance parameters. For example, ordinary Ge, despite its low absorption at 1550 nm, can be made into a high-responsivity detector by making it long in a waveguide implementation. However, the penalty for this is high dark current, large capacitance, and possibly, bandwidth reduction.
A reduction in the capacitance typically associated with conventional silicon applications has been realized using silicon-on-insulator (SOI) structures. As the name suggests, SOI refers to the use of a layered silicon-insulator-silicon substrate with the silicon junction being above the electrical insulator and isolated from the bulk silicon to provide a lower parasitic capacitance. However, the epitaxial silicon typically used in the SOI structure is less than desirable for many applications. For instance, silicon exhibits a relatively high and indirect bandgap, and thus, has a poor response to light having certain wavelengths, such as wavelengths (near) infrared wavelengths. Germanium is an example material that can be a desirable alternative to silicon for a variety of applications. For instance, germanium is a promising channel material for MOS-type transistors due to this high carrier mobility. Germanium also has other material properties that differ from silicon, such as a smaller bandgap. These properties facilitate optoelectronic devices and many additional device options. In the past few decades, investigations have been conducted regarding the use of non-silicon materials, such as germanium, for integrated circuit applications due to their enhanced qualities relative to other types of semiconductor materials, such as silicon.
Single-crystal materials are desirable for use in active regions due to their characteristics relative to, for example, polycrystalline materials. However, single-crystal materials, such as germanium, are difficult to manufacture on a silicon platform including the bulk Si and SOI wafers. Lattice mismatch may exist between any two different types of crystalline materials. For instance, one way to grow germanium is to directly use epitaxy methods at a seed interface that includes silicon, a lattice mismatch (e.g., about 4 percent) between the germanium and silicon can result in compressive stress on the germanium. Moreover, such an approach of directly growing pseudomorphically strained Ge on Si is limited to thin films of Ge (e.g., on the order of only a few nanometers thick), because pure Si (4 percent) limits the thickness to only a few nanometers before defects are realized. For many applications, a much larger thickness is desirable because it would result in increased absorption of light. Approaches to reducing the bandgap and/or creating a direct bandgap in germanium, such as those using a virtual substrate as taught by U.S. Pat. No. 6,897,471 to Soref et al., have been relatively complex and difficult to use.
Some other techniques for creating thicker germanium films require the films to be in contact with silicon. Moreover, such techniques often also require a prolonged anneal step at high temperature (e.g., around 900° C.) after growing Ge to anneal the defects. This, in general, eats up a lot of thermal budget for other CMOS devices, thus rendering a non-seamless integration with CMOS flow. In particular, a prolonged high temperature anneal leads to diffusion of silicon into germanium at the interface, which dilutes germanium concentration, increasing its bandgap, and reducing absorption efficiency.
Thus, these and other critical issues of desired band-structure modification of Ge as well as the growth of thick and thin Ge films in a true silicon and CMOS compatible manner and other aforementioned issues have presented challenges to the implementation and design of efficient light communicator circuits. Accordingly, there is a need for active optical devices that are easy to manufacture on a silicon platform and that provide efficient light-emission and amplification, modulation, and high responsitivity/uniform detection for wavelengths used in communication standards.